Power semiconductors, as distinguished from signal semiconductors, are used to process and control the flow of electric energy supplied to user loads. The utility of such devices is driven by their ability to quickly and efficiently switch on and off large operating voltages and currents. Power semiconductor switching devices are increasingly being designed to handle applications requiring high blocking voltages in the off condition, typically 1 kV and greater, and high current requirements in the on state, typically 1A and greater. Recent advances in device operating thresholds, however, have imposed operational and fabrication-related problems for power semiconductor devices.
Historically, power semiconductor devices have required large switching currents to handle the corresponding high device currents. Large switching currents result in device inefficiencies since excessive electrical power is required to operate the device. Power semiconductor devices to-date have employed metal-oxide-semiconductor (MOS) gate structures in a variety of arrangements to achieve the low current turn-on and turn-off requirements of these devices. However, MOS gates have experienced operational and fabrication-related reliability problems as the operational boundaries of the power semiconductor devices have been expanded. In particular, the high operating device voltages create large electric fields within these devices, which poses long-term reliability problems for the oxides used in the MOS gates. Trenched MOS gates (UMOS), as found in the paper by A. K. Agarwal et al. entitled SiC Power Device Development given at the All Electric Combat Vehicle (AECV) Second International Conference 8th-12th June 1997, and buried structures, U.S. Pat. No. 5,543,637, have been employed to partially overcome these oxide limitations. In each of these arrangements, however, large electric fields are still present at the oxide interfaces thereby compromising the long-term oxide reliability. Finally, gate oxides are often fabricated on implanted semiconductor regions, which results in low oxide quality and reliability, particularly in power devices fabricated from SiC. An exemplary high-power thyristor device employing such a MOS gate structure can be found in the above article by A. K. Agarwal.
The need exists for monolithic, simply constructed, easily fabricated, power semiconductor devices in which the controlling gate structures are removed from the large electric fields within the device. Although non-oxide gate structures are preferred, the need also exists to provide a reliable, non-implanted semiconductor surface on which to fabricate gate oxides, for those power semiconductors which continue to employ MOS gates, and to isolate such gate oxide from the large electric field stresses.